Structure and superconductivity in lithium-intercalated niobium dichalcogenides

The series Li/sub x/NbS2 (0 less than or equal to x less than or equal to 0.50) and Li/sub x/NbSe2 (0 less than or equal to x less than or equal to 0.67) have been prepared by high-temperature reaction of Li2S, Nb, and S or Li2Se, Nb, and Se. Structures have been determined by analysis of X-ray powder patterns with polytypism observed in the sulfide series. Evidence of staging and Li ordering is reported, and crystal parameters are proposed for a second-staged sample, Li/sub 0.2/NbSe2. In the series Li/sub x/NbSe2,T/sub c/ exhibits a monotonic decrease with increasing lithium content, falling to less than 2 K by x = 0.25. In contrast, in the Li/sub x/NbS2 series T/sub c/ exhibits a more complicated pattern with T/sub c/ plummeting to below 2 K by x = 0.05, only to rebound to 3.7 K at x = 0.33 and then fall to below 2 K by x = 0.4. 18 references, 8 figures, 4 tables